1. Field of the Invention
The invention relates to an implantable prosthesis. In particular, the invention relates to endoluminal grafts and stent-grafts which are deployed in a blood vessel which has a varying diameter. The invention is particularly suited for repairing the aortic artery and daughter arteries, although it is not limited thereto.
2. State of the Art
An endoluminal stent-graft typically includes tubular graft material which is affixed to the inside or outside of a woven metallic stent and is delivered to the damaged site of a blood vessel via a catheter. Endoluminal stent-grafts are most often used to repair blood vessels affected by a variety of lesions such as stenoses or aneurysms. A typical prior art stent, shown in FIGS. 1-6, is a metallic structure 10 made of braided wire 12 such as stainless steel, cobalt-chromium-nickel super alloys and combinations, co-extrusions or braised combinations of the above with tantalum, gold, platinum and the like. Stents are also made from memory alloys such as nitinol and the like. Typical stents are disclosed in U.S. Pat. Nos. 4,655,771 and 4,954,126 to Wallsten, the complete disclosures of which are hereby incorporated herein by reference, and in U.K. Patent Number 1,205,743 to Didcott, the complete disclosure of which is also hereby incorporated herein by reference. Generally, the wires 12 are braided with a large pick size, i.e. with relatively large interstices 14 between the wires, so that axial expansion of the stent causes a diametrical compression of the stent. Most often the braiding and/or the metal chosen for the wires yields a resilient stent which is self-expanding. However, some stents are not self-expanding and are expanded with the use of a balloon catheter. In the case of self-expanding stents, the proximal and distal ends 16, 18 of the stent are usually flared when expanded.
While endoluminal stents have been used without any graft material when repairing stenoses, it is now generally preferred to use a graft material in combination with the stent when repairing stenoses as well as when repairing aneurysms. The graft material most often used in endoluminal grafts is a PET or polytetrafluroethylene (PTFE) material which is folded to reduce its size and which is attached to one or both ends of a radially expandable stent by means of sutures. When the stent self-expands or is balloon expanded, the graft unfolds around the stent. The above-referenced parent application discloses a stent-graft which incorporates an improved self-expanding graft material.
While the primary use of endoluminal stents is to treat stenoses, stents are also sometimes used in conjunction with graft material to bridge aneurysms. The advantage of using a stent in bridging aneurysms is that the expanded stent helps to fix the graft in place, can eliminate the need for sutures, and may provide some additional resistance to hoop stress. Prior art FIGS. 2-5 illustrate the deployment of a stent-graft to bridge an aneurysm.
Referring now to FIGS. 2-5, the ends of the stent 10 are axially displaced inside an introducer 20 which includes an inner catheter 22 having a soft (dilator) tip 24 and an outer sheath 26. The introducer 20 is delivered through a blood vessel 28 with the aid of a guide wire 30 which is inserted through the lumen of the inner catheter 22. The introducer 20 is guided over the guide wire 30 to the site of an aneurysm, in this case two adjacent aneurysms, namely distal aneurysm 32 and proximal aneurysm 34. With the aid of fluoroscopy, the introducer 20 is positioned so that the soft tip 24 is located distally relative to the distal aneurysm 32. The outer sheath 26 is drawn proximally while the inner catheter 22 is held stationary. This releases the distal end 18 of the stent 10 which self-expands to the inner diameter of the vessel 28 as shown in FIG. 3. Continued proximal movement of the outer sheath 26 releases the remainder of the stent 10 as shown in FIG. 4 until the proximal end 16 of the stent 10 expands to the inner diameter of the vessel 28 proximal of the proximal aneurysm 34 as shown in FIG. 5, after which the introducer 20 and the guide wire 30 are removed from the vessel 28.
From the foregoing, it will be appreciated that by using an appropriately sized stent-graft, the aneurysms 32, 34 in FIGS. 2-5 are effectively bridged utilizing the procedure described above. In particular, the stent-graft must be long enough so that its proximal and distal ends extend beyond the aneurysms and expand into healthy areas of the blood vessel. Moreover, the stent-graft must be chosen to have the appropriate expanded diameter so that a good seal is made between the stent-graft and the inner wall of the blood vessel. However, the diameter should not be so large that when the stent expands, the outward pressure of the expanding stent damages the wall of the blood vessel.
Because of the above considerations, it is difficult or impossible to bridge an aneurysm with a stent-graft when the diameter of the blood vessel on either side of the aneurysm differs by any significant amount. For example, as shown in FIG. 6, the distal end 18 of a stent-graft 10 is greatly compressed as compared to the proximal end 16 when the stent-graft is used to bridge aneurysms 32, 34 where the diameter of the vessel 28 on the proximal side 28a of the aneurysms 32, 34 is substantially greater than the diameter of the vessel on the distal side 28b of the aneurysms 32, 34. Depending on the nature of the particular stent-graft, this can cause damage to the vessel on the distal side 28b or can result in an inward tapering of the distal end 18 of the graft to a xe2x80x9ccigar shapexe2x80x9d. In the former situation, the damage can result in an additional aneurysm or rupture of the vessel. In the latter situation, the distal end 18 of the graft can obstruct the flow of blood, or jeopardize the seal between the distal end 18 and the inner wall of the vessel 28b. In the case of obstruction, occlusion of the vessel may occur which can be catastrophic to the patient. In the case of seal weakening, blood will enter into the aneurysmal sac and promote continued growth of the aneurysm.
More often than not the vessels of the vascular tree especially in the abdominal aortic artery exhibit the joining of vessels having very different diameters. For example, as shown in FIG. 7, the abdominal aortic artery 50 is the trunk from which the renal arteries, right 52, left 54 and the iliac arteries, right 56, left 58 proceed. An aortic aneurysm 60 between the renal arteries and the iliac arteries is very difficult to bridge since the diameter of the aortic artery is approximately 25 mm, whereas the diameter of the iliac artery is about 12 mm. A stent-graft having a diameter of 27 mm will fit well in the aortic artery, but will be too large for the iliac artery. A 13 mm diameter stent-graft will fit well in the iliac artery, but will be too small for the aortic artery.
The above-referenced parent application discloses a bifurcated stent-graft which is useful in repairing an abdominal aortic aneurysm and iliac aneurysm. The bifurcated graft is located in the abdominal aortic artery just above the iliac arteries with its bifurcated end closest to the iliac arteries. The bifurcated stent-graft effectively bypasses an aneurysm in the aortic artery and provides a radiopaque bifurcated guide to the iliac arteries. Once the bifurcated graft is deployed, an additional graft may be deployed in each of the iliac arteries. The additional grafts are deployed through the legs of the bifurcated stent-graft. The bifurcated legs provide separate fluid couplings for the two additional grafts so that blood can flow from the aortic artery to both iliac arteries.
Subsequent to the development of the bifurcated stent-graft of the parent application, additional discoveries have been made regarding the use of multiple stent-grafts to bridge vessels of different diameter. In particular, it is sometimes desirable to bridge the aortic artery with only one of the iliac arteries.
In addition, it has been discovered that in some situations where a stent-graft has been implanted to bridge an aneurysm, the stent-graft will continue to expand radially long after the time of implantation. This is particularly likely where there is continuous progression of aneurysmal disease and dilation of the neck of the aneurysm. The continued radial expansion of the stent-graft results in a continued axial shortening of the stent-graft which often results in the ends of the stent-graft becoming dislodged from the blood vessel whereupon the prosthesis floats free inside the aneurysm causing serious danger to the patient.
It is therefore an object of the invention to provide endoluminal stent-grafts which are useful for bridging vessels of different diameter.
It is also an object of the invention to provide methods for using endoluminal stent-grafts to bridge vessels of different diameter.
It is still another object of the invention to provide an endoluminal stent-graft with a limited radial expandability and limited axial compressibility.
In accord with these objects which will be discussed in detail below, the modular endoluminal stent-grafts of the present invention include at least two different sized stent-grafts which are deployed one within the other. According to one embodiment of the invention, a first stent-graft is provided having a flared end which is expandable to a first diameter and a midsection which is expandable to a second diameter smaller than the first diameter. A second stent-graft is also provided having an end which is expandable to a diameter which engages the midsection of the first stent-graft. The first embodiment of the invention is deployed by expanding the first stent-graft such that its flared end engages a large diameter vessel, then expanding the second stent-graft inside the midsection of the first stent graft and inside a small diameter vessel such that the second stent graft engages the small diameter vessel and the midsection of the first stent-graft. Both the first and second stent-grafts may be manufactured in a conventional manner using conventional materials. According to a second embodiment of the invention, the midsection of the first stent-graft is reinforced with a flexible member to restrict the midsection from ballooning due to the outward pressure of the second stent-graft deployed within the lumen of the first stent-graft. The reinforcing member may be applied to all or a portion of the stent-graft. The reinforcing member is also useful in preventing the stent-graft from ballooning due to the presence of static blood pressure over time after implantation.
According to other aspects of the invention, the first stent-graft is provided with two flared ends and the second stent graft is provided with or without flared ends.
According to still another embodiment of the invention, three or more stent-grafts of different expanded diameter are deployed one within the other.
According to another embodiment of the invention, two or more stent-grafts of different diameter are pre-coupled to each other prior to deployment and are deployed using a single introducer in substantially one step.
According to still other aspects of the invention, the second and/or third stent-grafts are reinforced with a flexible member to restrict the midsection from ballooning.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.